It is common knowledge in the oil industry that the introduction of heat into an oil reservoir, especially a reservoir containing heavy or waxy oil, is beneficial. There are several methods used to achieve reservoir heating. They include steam injection, in situ combustion and electrical heating. The present invention is concerned with electrical heating.
Electric heating can take the form of resistance heating or induction heating. The present invention is concerned with induction heating. A significant drawback of the non-conductive heating approach is that high non-conductive heating element temperatures can cause coking, scaling and other forms of deposition, which raise the thermal resistance through which heat flows from the non-conductive element to the well bore. This elevated thermal resistance either increases the operating temperature of the non-conductive element for the same power level, or reduces the operational power level for the same element temperature.
Electric heating may also be classified by the method of conveying electric power to the downhole heater. In both non-conductive and inductive heating, electric power may be conveyed downhole via an isolated production tubing string. The present invention is concerned with inductive heating, in which electrical power is conveyed downhole via a cable running from the surface power system to the downhole tool.
Induction heating tools may be run into the wellbore of an existing well on a tubing string. The induction-heating tool may be landed opposite an interval to be heated. The tool can readily be removed for repair. There is no need for a permanent modification to the well to facilitate heating, such as the incorporation of isolators into the casing string, which is the case for some electrical heating systems which use the tubing string as an electrical conductor.
The tool itself comprises a transformer-type core-coil assembly jacketed in a tubular housing. Each core-coil assembly comprises a conductive wire coil wound on a magnetically permeable, laminated core. AC power is supplied to the coil from a power source at ground surface through a bus extending down the wellbore. Application of power to the coil induces eddy currents in the adjacent steel casing or screen liner, thereby increasing its temperature. The hot casing or liner in turn heats the near-wellbore region of the reservoir and oil within the wellbore. The term "casing" as used herein broadly means casing, sand exclusion liners and similar metal tubular goods having an interior flow path that defines the well bore.
Canadian Patent Application No. 2,208,197, filed by R. E. Isted and published Dec. 18, 1998, discloses an induction-heating tool. Although the application discloses a stainless steel, magnetically-transparent housing, the housing is not electro-magnetically transparent and a high tool winding operating temperature can be expected to reduce the tool operating life.
Canadian Patent 2,090,629, issued to J. E. Bridges on Dec. 29, 1998 discloses another induction heating tool. The '629 patent discloses a method of conveying electrical power via the production tubing. This method requires modification of the well casing for the installation of an electrically non-conductive window. This well modification is expensive and likely to be the source of serious reliability concerns.
The visco-skin effect, which reduces oil recovery, arises when heavy oil, approaching the wellbore, loses light ends due to changing pressure conditions, leaving a heavier oil clogging the reservoir immediately adjacent the wellbore. As previously stated, the hot casing heats both the near-wellbore region of the adjacent reservoir and the oil entering or within the casing. This has the benefits of ameliorating the visco-skin effect and improving the production and pumpability of the oil. The application of heat in this manner thus can stimulate and significantly improve the production rates of high viscosity heavy oil and waxy wells.